1. Mix in a small bowl: tuna, mayo, mustard, celery, onion, and peppers.
2. Butter one side of each slice of bread.
3. Place the bread butter side down on a skillet over medium low heat.
4. Spoon the tuna mixture on top and top with cheese.
5. After the bread is golden brown on the bottom, place the second slice on top and flip, again butter side down.
6. Remove when the bottom is golden brown and the cheese is gooey.